Nonaqueous electrolyte batteries (mainly a lithium-ion secondary battery), whose negative electrode active material is a carbonaceous material and whose positive electrode active material is a sheet oxide containing nickel, cobalt, manganese etc., have already been in practical use as power sources in a wide range of fields from small things such as various electronic devices to large things such as electric vehicles. There is a strong demand from users for a smaller, lighter battery that can be used for a long period of time and has a longer life, and it has been strongly desired that the density of battery capacity be further improved and the ability of repeated use be improved. However, conventional carbonaceous materials are limited as to improving charge-discharge capacity. Furthermore, carbon baked at low temperatures, which is regarded as having a high capacity, has a low mass density and therefore its charge-discharge capacity per unit volume is difficult to increase. Therefore, for realizing a high-capacity battery, a new negative electrode material needs to be developed.
It has been proposed using, as a negative electrode material that can achieve a higher capacity than a carbonaceous material, a single-component metal such as aluminum (Al), silicon (Si), germanium (Ge), tin (Sn) or antimony (Sb). In particular, the use of Si as a negative electrode material can give a high capacity as high as 4200 mAh per unit weight (1 g). However, with the negative electrodes made from these single-component metals, the repeated uptake and release of Li causes elements to change to microparticles. Therefore, it is not possible to obtain a high charge-discharge cycle property.
For the purpose of solving these problems, amorphous tin oxide and silicon oxide etc. are able to achieve both an increase in capacity and a high cycle property. A further improvement can be achieved by using a carbon material in combination with the amorphous tin oxide or silicon oxide etc., as shown in Patent Literature 1. On the other hand, even with the use of the improved high-capacity tin oxide or silicon oxide, volume expansion during charging and shrinkage during discharging still impose a significant load on the battery. Specifically, for example, copper foil used as a current collector greatly deforms and an internal short-circuit is likely to occur when the battery is first charged, or the copper foil develops holes due to repeated use. This seriously compromises safety.